1. Field of the Invention
The present invention provides chimeric IBDV immunogens which actively protect against virulent and lethal challenge by Classic and variant IBDV strains, and methods for obtaining vaccines containing these chimeric immunogens and vaccines.
2. Discussion of the Background
Infectious bursal disease virus (IBDV) is responsible for a highly contagious immunosuppressive disease in young chickens which causes significant losses to the poultry industry worldwide (reviewed in Kibenge (1988) "J. Gen. Virol.", 69:1757-1775). Infection of susceptible chickens with virulent IBDV strains can lead to a highly contagious immunosuppressive condition known as infectious bursal disease (IBD). Damage caused to the lymphoid follicles of the bursa of Fabricius and spleen can exacerbate infections caused by other agents and reduce a chicken's ability to respond to vaccination as well (Cosgrove (1962) "Avian Dis.", 6:385-3894).
There are two serotypes of IBDV (McFerran et al (1980) "Avian Path.", 9:395-404). Serotype 1 viruses are pathogenic to chickens and differ markedly in their virulence (Winterfield et al (1978) "Avian Dis.", 5:253-260), whereas serotype 2 viruses, isolated from turkeys, are avirulent for chickens (Ismail et al (1988) "Avian Dis.", 32:757-759; Kibenge (1991) "Virology", 184:437-440).
IBDV is a member of the Birnaviridae family and its genome consists of two segments of double-stranded RNA (Dobos et al (1979) "J. Virol.", 32:593-605). The smaller segment B (.about.2800 bp) encodes VP1, the dsRNA polymerase. The larger genomic segment A (.about.3000 bp) encodes a 110 kDA precursor polyprotein in a single open reading frame (ORF) that is processed into mature VP2, VP3 and VP4 (Azad et al (1985) "Virology", 143:35-44). From a small ORF partly overlapping with the polyprotein ORF, segment A can also encode VP5, a 17 kDa protein of unknown function (Kibenge et al (1991) "J. Gen. Virol.", 71:569-577).
While VP2 and VP3 are the major structural proteins of the virion, VP2 is the major host-protective immunogen and causes induction of neutralizing antibodies (Becht et al (1988) "J. Gen. Virol.", 69:631-640; Fahey et al (1989) "J. Gen. Virol.", 70:1473-1481). VP3 is considered to be a group-specific antigen because it is recognized by monoclonal antibodies (Mabs) directed against VP3 from strains of both serotype 1 and 2 (Becht et al (1988) "J. Gen. Virol.", 69:631-640). VP4 is a virus-coded protease and is involved in the processing of the precursor protein (Jagadish et al (1988) "J. Virol.", 62:1084-1087).
In the past, control of IBDV infection in young chickens has been achieved by live vaccination with avirulent strains, or principally by the transfer of maternal antibody induced by the administration of live and killed IBDV vaccines to breeder hens. Unfortunately, in recent years, virulent variant strains of IBDV have been isolated from vaccinated flocks in the United States (Snyder et al (1988) "Avian Dis.", 32:535-539; Van der Marel et al (1990) "Dtsch. Tierarztl. Wschr.", 97:81-83). The use of a select panel of Mabs, raised against various strains of IBDV, has led to the identification of naturally occurring GLS, DS326, RS593 and Delaware variant viruses in the United States. Substantial economic losses have been sustained due to the emergence of these antigenic variants (Delaware and GLS) in the field (Snyder et al (1992) "Arch. Virol.", 127:89-101, U.S. application Ser. No. 08/216,841, filed Mar. 24, 1994, Attorney Docket No. 2747-053-27, Snyder, now abandoned). These variant strains are antigenically different from the Classic strains of IBDV most typically isolated before 1985, and lack epitope(s) defined by neutralizing monoclonal antibodies (Mabs) B69 and R63 (Snyder et al (1988) "Avian Dis.", 32:527-534; Snyder et al (1988) "Avian Dis.", 32:535-539; Snyder et al (1992) "Arch. Virol.", 127:89-101). Since the appearance of these variant strains in the field, many commercially available live and killed vaccines for IBDV have been reformulated in an attempt to better match the greater antigenic spectrum of viruses recognized to be circulating in the field.
Efforts to develop a recombinant vaccine for IBDV have been made, and the genome of IBDV has been cloned (Azad et al (1985) "Virology", 143:35-44). The VP2 gene of IBDV has been cloned and expressed in yeast (Macreadie et al (1990) "Vaccine", 8:549-552), as well as in recombinant fowlpox virus (Bayliss et al (1991) "Arch. Virol.", 120:193-205). When chickens were immunized with the VP2 antigen expressed from yeast, antisera afforded passive protection in chickens against IBDV infection. When used in active immunization studies, the fowlpox virus-vectored VP2 antigen afforded protection against mortality, but not against damage to the bursa of Fabricius.
Recently, the synthesis of VP2, VP3 and VP4 structural proteins of the variant GLS IBDV strain in a baculovirus expression system has been described (Vakharia et al (1993) "J. Gen. Virol.", 74:1201-1206). In an initial two dose active immunity study in SPF chickens, baculovirus expressed GLS proteins were able to confer 79% protection against virulent GLS challenge (Vakharia et al (1993) "J. Gen. Virol.", 74:1201-1206). In a subsequent extended study of active cross-immunity, by increasing the antigenic mass of the baculovirus expressed GLS protein, complete protection against the variant GLS and E/Del strains was obtained with a single dose, but only partial protection was afforded against the Classic STC strain unless two doses were administered.
In recent years, the complete, nucleotide sequences of the large segment A of five serotype 1 IBDV strains; 002-73 (Hudson et al (1986) "Nucleic Acids Res.", 14:001-5012), Cu-1, PBG98, 52/70 (Bayliss et al (1990) "J. Gen. Virol.", 71:1303-1312), STC (Kibenge (1990) "J. Gen. Virol.", 71:569-577), and serotype 2 OH strain (Kibenge (1991) "Virology", 184:437-440) have been determined. In addition, the VP2 gene of virulent Japanese IBDV strains (Lin et al (1993) "Avian Dis.", 37:315-323) and Delaware variants A and E (Lana et al (1992) "Virus Genes", 6:247-259; Heine et al (1991) "J. Gen. Virol.", 22:1835-1843) have been sequenced. However, noone has completely cloned and characterized the entire long segment of any United States IBDV variant.